We should allow the browser to handle the content of <embed> frames
as we do for <object>. This would allow a mozilla with native SVG
to handle the SVG content of pages that were written for use with
a plugin.

The Adobe SVG demos, the Adobe "SVG Site Spotlight", and the W3C's SVG
test suite all are using <embed>.
Why would an embed of text/html be a bad thing? A roundabout way of
doing an iframe, but doesn't seem harmful.

Using the 'object' tag to embed SVG in web pages crashes Safari 1.0, so I have
been forced to switch to using 'embed' [1]. Since updates to Safari are not
available to users of Mac OS X version 10.2, I have to assume Safari 1.0 will be
in circulation for some years to come.
[1]: http://www.alleged.org.uk/pdc/2004/02/14.html
For some reason, almost all browsers have had disastrously buggy implementations
of 'object' at some point; Safari is merely the most recent. As a result,
'embed' is a lot more prevelant than the W3C would have liked.

Some standard-conscious people are forced to use 'embed' because MS IE refuses
to 'render' the following straightforward construct using 'object' complaining
about 'insecure' content being fed to ActiveX control.
<object data="test.mp3" type="audio/mpeg" height="239" width="45">
You are expected to listen to ..... </object>
To avoid that, one has to use a construct like this:
<object id="Player" CLASSID="CLSID:6BF52A52-394A-11d3-B153-00C04F79FAA6"
width="239" height="45" title="Playing Music">
<param name="URL" value="music.mp3" />
<param name="autoStart" value="true" />
<embed src="music.mp3" type="application/x-mplayer2"
width="239" height="45"
pluginspage = "http://www.microsoft.com/Windows/MediaPlayer/">
<noembed>Music to be played</noembed>
</object>
Again, those caring about the standard compliance would love to use 'object' in
place of 'embed', but MS IE does NOT ignore the inner 'object' and complain
about 'the security' although it should just ignore it because it can process
the outer object.

Comment on attachment 164791[details][diff][review]
only allow handing of svg <embed>
>+#ifndef MOZ_SVG
> // only do the following for the object tag
> if (aContent->Tag() != nsHTMLAtoms::object)
> return rv;
>+#endif
Just drop this entirely. It makes things more confusing, and it's not
performance-critical.
Also, while you're there, it looks like IsSupportedImage and
IsSupportedDocument can be static. If that's correct, please make them static
(one less argument passed).